1. Field of the Invention
The present invention relates to a semiconductor device including an oxide semiconductor.
In this specification, the semiconductor device refers to all devices that can operate by utilizing semiconductor characteristics. A transistor described in this specification is a semiconductor device, and an electrooptic device, a semiconductor circuit, and an electronic device which include the transistor are all semiconductor devices.
2. Description of the Related Art
Transistors used for most flat panel displays typified by a liquid crystal display device and a light-emitting display device are formed using silicon semiconductors such as amorphous silicon, single crystal silicon, and polycrystalline silicon provided over glass substrates. Further, transistors formed using such silicon semiconductors are used in integrated circuits (ICs) and the like.
Attention has been directed to a technique for using metal oxides exhibiting semiconductor characteristics for transistors, instead of the above silicon semiconductors. Note that in this specification, a metal oxide exhibiting semiconductor characteristics is referred to as “oxide semiconductor”.
For example, a technique is disclosed in which a transistor is formed using a Zn—O-based metal oxide or an In—Ga—Zn—O-based metal oxide as an oxide semiconductor and such a transistor is used as a switching element or the like in a pixel of a display device (see Patent Documents 1 and 2).
Further, a technique is disclosed in which in a transistor including an oxide semiconductor, a highly conductive oxide semiconductor containing nitrogen is provided as buffer layers between a source region and a source electrode and between a drain region and a drain electrode, and thereby the contact resistance between the oxide semiconductor and the source electrode and the contact resistance between the oxide semiconductor and the drain electrode are reduced (see Patent Document 3).
Further, a technique for forming a channel formation region, a source region, and a drain region in a self-aligned manner in a top-gate transistor including an oxide semiconductor is disclosed (see Non-Patent Document 1).